设计模式学习专栏五--------命令模式-白红宇

设计模式学习专栏五--------命令模式

场景

设计一个智能遥控器,遥控器上有7个插槽 , 每个插槽(某设备具体供应商)对应两个按钮 on , off ,以及一个全局的撤销操作undo.

较差的实现方式

判断每个插槽对应的具体厂商是谁, 然后做出对应的动作

if(slot1 == Light){    light.on()}else if(slot1 == Hottub){    hottub.on()}else if(slot1 == TV){    tv.on();}...复制代码

出现的问题
- 遥控器和具体的设备厂商耦合到了一起 , 遥控器需要认识某个插槽当前对应设备的厂商
- 当加入/删除新的设备厂商时, 当前代码都需要进行改动

如何解决

对象村餐厅的例子

一张订单封装了准备餐点的请求

女招待的工作是接收订单 , 然后调用订单的OrderUp()方法 , 女招待不需要担心订单的内容是什么, 或者由谁来准备餐点 , 她只需要指导,订单由一个OrderUp()方法可以调用即可.

快餐厨师具有准备餐点的知识 . 他只要看到订单就知道如何准备餐点 . 厨师和女招侍之间从不需要直接沟通.

把餐厅想象成设计模式的一种模型 , 这个模型允许通过**"封装请求的命令"(订单)**将 "发出请求的对象"(女招待, "订单来啦") 和 "接收与执行这些请求的对象"(厨师) 分隔开 .

从餐厅到命令模式

命令模式总览

定义: 将请求封装成对象(订单),将发出请求的对象(服务员/按钮)和执行请求的对象(厨师/具体供应商)解耦 , 也可以支持撤销操作

类图

模式的理解
- 角色
  - 封装请求的命令对象(订单) : 一个命令对象通过在特定接收者上绑定一组动作来封装一个请求 receiver.action1(), 该对象之暴露一个execute()方法, 当此方法被调用时, 接收者就会进行对应的动作 . 从女招待的角度来看, 它不需要知道具体哪个接收者进行了什么动作,只知道如果调用execute()方法 , 请求的目的就能达到了
  - 发出请求的对象(女招待) : ①负责接收命令对象 ②在合适的时候调用命令对象的execute()方法
  - 执行请求的对象(厨师) : 真正执行请求的接收者
- 细节
  - "撤销操作" 只需在Invocker中定义变量Command undoCommand保存撤销操作即可 . 如果要联系撤销, 则可以用栈的形式存储执行过的命令
```
public class LightOnCommand implements Command {	Light light;	public LightOnCommand(Light light) {		this.light = light;	}	public void execute() {		light.on();	}	public void undo() {		light.off();	}}复制代码
```
```
public class RemoteControlWithUndo {	Command[] onCommands;	Command[] offCommands;	Command undoCommand;	//记录上一次命令 	public RemoteControlWithUndo() {		onCommands = new Command[7];		offCommands = new Command[7]; 		Command noCommand = new NoCommand();		for(int i=0;i<7;i++) {			onCommands[i] = noCommand;			offCommands[i] = noCommand;		}		undoCommand = noCommand;	}  	public void setCommand(int slot, Command onCommand, Command offCommand) {		onCommands[slot] = onCommand;		offCommands[slot] = offCommand;	} 	public void onButtonWasPushed(int slot) {		onCommands[slot].execute();		undoCommand = onCommands[slot];	//记录上一次命令	} 	public void offButtonWasPushed(int slot) {		offCommands[slot].execute();		undoCommand = offCommands[slot];	//记录上一次命令	} 	public void undoButtonWasPushed() {		undoCommand.undo();	//执行撤销命令	}}复制代码
```
  - "Party模式" : 封装一次请求中的批量操作 (按下一个按钮,同时弄暗灯光 , 打开音响和电视,设置好DVD)
```
public class MacroCommand implements Command {	Command[] commands; 	public MacroCommand(Command[] commands) {		this.commands = commands;	} 	public void execute() {		for (int i = 0; i < commands.length; i++) {			commands[i].execute();		}	}}复制代码
```
- 使用场景
  - 队列请求 (日志安排 , 线程池 , 工作队列)
    - 以线程池为例 , 将线程需要执行的动作封装进Runnable对象中
    - 线程池以阻塞队列存储Runnable对象
    - 当线程空闲时 , 从队列中取出Runnable对象 , 并执行Runnable中重写的Run()方法
    - 总结 : 即线程池中的线程不需要知道具体的执行者是谁,干了什么,只需要在空闲时从队列中取出命令对象Runnable并调用run()方法即可
  - 日志请求
    - 当每个命令被执行时, 会被储存在磁盘中
    - 在系统死机后, 重新加载存储的命令,并以准备的次序执行

案例代码部分

命令对象Command

public interface Command {	public void execute();	public void undo();}复制代码

具体命令对象--开灯

public class LightOnCommand implements Command {	Light light;	int level;	public LightOnCommand(Light light) {		this.light = light;	} 	public void execute() {        level = light.getLevel();		light.on();	} 	public void undo() {		light.dim(level);	}}复制代码

具体命令对象 --关灯

public class LightOffCommand implements Command {	Light light;	int level;	public LightOffCommand(Light light) {		this.light = light;	} 	public void execute() {        level = light.getLevel();		light.off();	} 	public void undo() {		light.dim(level);	}}复制代码

设备厂商 -- Receiver

public class Light {	String location;	int level;	public Light(String location) {		this.location = location;	}	public void on() {		level = 100;		System.out.println("Light is on");	}	public void off() {		level = 0;		System.out.println("Light is off");	}	public void dim(int level) {		this.level = level;		if (level == 0) {			off();		}		else {			System.out.println("Light is dimmed to " + level + "%");		}	}	public int getLevel() {		return level;	}}复制代码

调用者Invoker --远程遥控器

public class RemoteControlWithUndo {	Command[] onCommands;	Command[] offCommands;	Command undoCommand; 	public RemoteControlWithUndo() {		onCommands = new Command[7];		offCommands = new Command[7]; 		Command noCommand = new NoCommand();		for(int i=0;i<7;i++) {			onCommands[i] = noCommand;			offCommands[i] = noCommand;		}		undoCommand = noCommand;	}  	public void setCommand(int slot, Command onCommand, Command offCommand) {		onCommands[slot] = onCommand;		offCommands[slot] = offCommand;	} 	public void onButtonWasPushed(int slot) {		onCommands[slot].execute();		undoCommand = onCommands[slot];	} 	public void offButtonWasPushed(int slot) {		offCommands[slot].execute();		undoCommand = offCommands[slot];	} 	public void undoButtonWasPushed() {		undoCommand.undo();	}}复制代码

主程序

public class RemoteLoader {	public static void main(String[] args) {		RemoteControlWithUndo remoteControl = new RemoteControlWithUndo();		Light livingRoomLight = new Light("Living Room");		LightOnCommand livingRoomLightOn =				new LightOnCommand(livingRoomLight);		LightOffCommand livingRoomLightOff =				new LightOffCommand(livingRoomLight);		remoteControl.setCommand(0, livingRoomLightOn, livingRoomLightOff);		remoteControl.onButtonWasPushed(0);		remoteControl.offButtonWasPushed(0);		System.out.println(remoteControl);		remoteControl.undoButtonWasPushed();	//撤销操作		System.out.println(remoteControl);		//此时应该是开灯状态	}}复制代码

输出结果

Light is onLight is off------ Remote Control -------[slot 0] headfirst.designpatterns.command.undo.LightOnCommand    headfirst.designpatterns.command.undo.LightOffCommand[slot 1] headfirst.designpatterns.command.undo.NoCommand    headfirst.designpatterns.command.undo.NoCommand[slot 2] headfirst.designpatterns.command.undo.NoCommand    headfirst.designpatterns.command.undo.NoCommand[slot 3] headfirst.designpatterns.command.undo.NoCommand    headfirst.designpatterns.command.undo.NoCommand[slot 4] headfirst.designpatterns.command.undo.NoCommand    headfirst.designpatterns.command.undo.NoCommand[slot 5] headfirst.designpatterns.command.undo.NoCommand    headfirst.designpatterns.command.undo.NoCommand[slot 6] headfirst.designpatterns.command.undo.NoCommand    headfirst.designpatterns.command.undo.NoCommand[undo] headfirst.designpatterns.command.undo.LightOffCommandLight is dimmed to 100%		//撤销操作 ==> 开灯状态------ Remote Control -------[slot 0] headfirst.designpatterns.command.undo.LightOnCommand    headfirst.designpatterns.command.undo.LightOffCommand[slot 1] headfirst.designpatterns.command.undo.NoCommand    headfirst.designpatterns.command.undo.NoCommand[slot 2] headfirst.designpatterns.command.undo.NoCommand    headfirst.designpatterns.command.undo.NoCommand[slot 3] headfirst.designpatterns.command.undo.NoCommand    headfirst.designpatterns.command.undo.NoCommand[slot 4] headfirst.designpatterns.command.undo.NoCommand    headfirst.designpatterns.command.undo.NoCommand[slot 5] headfirst.designpatterns.command.undo.NoCommand    headfirst.designpatterns.command.undo.NoCommand[slot 6] headfirst.designpatterns.command.undo.NoCommand    headfirst.designpatterns.command.undo.NoCommand[undo] headfirst.designpatterns.command.undo.LightOffCommand复制代码

参考

书籍: HeadFirst设计模式

代码参考地址: